Frequency modulated wave generator



March 16', 1943. J. A RANKIN 2,314,161

FREQUENCY MODULATED WAVE GENERATOR Filed Nov. 19, 1940 INVENTOR eloluovillankcn BY wem ATTORNEY Patented Mar. 16, Q v

. FREQUENCY MODULATED WAVE I GENERATOR John A. Rankin, Port Washington, N. Y., assignor to Radio Corporation of America, a corporation r of Delaware Application November 19, 1940, Serial No. 366,229

Claims.

My present invention relates to signal generators for frequency modulated waves, and more particularly. to'a signal generator capable of producing frequency modulated waves of substantially uniform amplitude.

Frequency modulated wave broadcasting is assigned at present to the 513 to 50 megacycle band.

' Frequency modulated receivers are of the superheterodyne type, and employ an intermediate frequency channel in the 2 to 4 megacycle range.

' Hence, in receiver development work signal gen- -maximum deviation of 500 kilocycles is desirable.

By deviation, as used in this application, is meant the frequency departure of the modulated radio frequency carrier above or below its unmodulated value.

In addition to the aforesaid wide frequency deviation it is desirable to have signa1 generators capable of handling modulation frequencies up to 15,000 cycle, since frequency modulation receivers are largely of higher fidelity than conventional broadcast receivers. One of the problems in signal generators has been substantially to eliminate amplitude modulation from the output of the frequency modulation signal generator. Usually in the case of frequency modulation generators the amplitude modulation introducedduring the modulation process has been subsequently removed by some form of amplitude limiter.

Particularly in the case of a frequency modulation system of the type wherein quadra-" ture circuit is utilized in conjunction with the oscillator to produce frequency variation thereof,

amplitude modulation efiects are introduced by modulated wave generatorwith constant output voltage amplitude, and wherein the usual limiter network is dispensed with; the constant output voltage amplitude being secured by utilizing the modulating source itsel'fisubstantially to eliminate any amplitude modulation introduced in the oscillator voltage during the modulation process. Another important object of this invention is to provide, in general, a method, and means for, r

, frequency modulating oscillations produced by an oscillator network, and the modulated oscillations being maintained of substantial constant amplitude by utilizing the modulation source to counteract any amplitude modulation effects which may arise during the frequency modulation of the oscillator output. 7

Still another object of this invention may be stated to residev inthe provision of anoscillator network equipped with a reactance control tube whose function it is to frequency modulate the produced oscillations in correspondence with ap-' plied modulating audio frequency voltage, and theapplied modulating voltage being simultaneously employed to control an electron discharge device associated with the oscillator in a man-- Still other objects of my invention are to improve generally the efflciency and simplicity of frequency modulated wave generators, and more particularly to provide 'a frequency modulated signal generator with constant output voltage amplitude which is not only reliable in operation, but is economically manufactured and assembled. V

The novel features which! believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best'be understood by reference to the following description taken in connection withthe drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect. r

In the drawing: Fig. 1 shows a circuit diagram of invention,

Figs. 2 and 3 show respectively different modiflcation's of the invention. Referring now to Fig. 1 there is shown a tubel which may be-of the 6J5 type which is provided with a plate 2 connectedto asource of positive side of the blocking condenser is connected to the cathode lead 1 by the grid leak resistor 8. The tank circuit comprises the coil 9 whose low potential end is established .at ground potential.

a frequency modulated signal wave generator embodying the the oscillator circuit is a conventional Hartley type circuit. Oscillation voltage produced across the tank circuit 9-") may be taken off at the high potential end .of coil 9, and transmitted through coupling condenser H to frequency multipliers and to an output attenuator.

Frequency modulation is accomplished by th use of a quadrature circuit operating on the oscil lator to produce frequency variation thereof in correspondence with applied modulating audio frequency voltage. There is connected, for this purpose, in shunt with the tank circuit a resistor 2 and condenser l3 in series. The quadrature tube itself is designated by numeral l4, and may be a tube of the GAG! type. This tube has its cathode at ground potential, while its plate I5 is supplied with positive potential through a radio frequency choke coil IS. The screengrid of the tube is connected to the source of positive potential through a voltage reducing resistor IT. The control grid l8 of the quadrature tube is connected through the condenser l9 to the junction of resistor I2 and condenser |3.-

Hence, alternating voltage developed across condenser |3 is impressed on the grid l8.

The plate I5 is connected to the high potential end of coil 9 through a direct current blocking condenser 2|], and it will be noted that the plate to cathode impedance of quadrature tube I4 is connected in shunt across the tank circuit 9-10. The modulation voltage source is conventionally represented at 2| the. modulation voltage, which is anaudio source, being applied to grid l8 through'the radio frequency choke coil A direct current bias. source may be arranged in series between ground and the modulation voltage source so as to apply a desired negative bias to the control grid 8, the numeral 23 designating the grid bias source.

Frequency modulation of the generated oscillations is accomplished in the following manner. If resistor I2 is large in comparison with the reactance' of condenser |-3 at the oscillator frequency, the" current will be in phase with the tank voltage. However, the voltage across conoscillator amplitude with quadrature tube mutual of the audio voltage applied to the grid of a quadrature tube fails to produce a linear frequency deviation, amplitude distortion results. Employing an amplitude limiter subsequent to the oscillator causes the generation of harmonics of the fundamental radio frequency, and then it is necessary to utilize networks in the output circuit of the limiter to eliminate such harmonics. In accordance with my present invention a device is utilized to make the oscillator input uniform regardless of the amplitude of the modulating voltage applied to the grid of the quadrature tube. In other words, in accordance with the present invention the modulated oscillator voltage fed through condenser II has its frequency deviation proportional to the amplitude of the modulating voltage, while the frequency of the modulating voltage determines the rate of change of the predetermined tank frequency.

In order to accomplish this step of maintaining the modulated oscillation voltage free of amplitude variation, there is utilized a tube 24 whose cathode is established at ground potential. The plate 25 is connected to the plate I! of quadrature tube H. The grid 26 of tube 24 is connected by lead 21 and adjustable tap 21' to a resistor shunted across the secondary of transformer 21". The bias source 23" biases grid 26. The primary of 21" is across source 2|. It will now be seen that the plate to cathode impedance to tube 24 is connected in shunt across tank circuit 9-H). .Hence, the tube 24 acts as a varying load on the oscillator tank circuit when the grid 26 is energized with'audio voltage from source 2| "which is of the proper phase and amplitude. ,In other words, any amplitude modulation effects introduced by virtue of source 2| acting on tube I4 is substandenser I3 due to this current will lag the current plate of the quadrature tube is in turn connected backto thetank circuit. The plate current of tube |4 varies at radio frequency in response to tially compensated for by the action of the source 2| on the grid of tube 24. The result of this.

compensation is to insure .an output from the tank circuit which is substantially free from amplitude variation. It may be pointed out that for the audio modulation voltage source there may be employed a 400 cycle audio oscillator, this source being sufllcient to produce full deviation over the required bands of frequency. The action of tube 24 ofFig. 1 in eliminating the amplitude modulation caused by the fretube. As an audio modulation voltage is applied to the grid-of tube H the Gm thereof is-effecthe radio frequency voltage on the grid 8.

Since the radio frequency plate current is in lagging quadrature withthe voltage across the tank circuit, it is equivalent to connecting a shunt inductance across coil 9 thus modifying the oscillator frequency.

The magnitude of the reflected, or simulated,

shuntinductance'depends on'the mutual conductance of quadrature tube 4. Hence, if the mutual conductance is varied by the audio modulation' voltage from source 2|; the frequency of the oscillator is varied at. the audio rate thereby producing frequency modulation of the generated oscillation frequency. Any variation in the with the tank circuit zvoltage which when amplified by tube I4 appearsin shunt with the tank circuit and is resistive. Hence, this causes a damping of the tank circuit. This damping, as I well as the inductance that is shunted across the tank circuit, varies in amplitude as dictated by the audio-modulation applied-to the grid of tube l4. The inductive component results in a frequency shift of the oscillator frequency, while the resistive component results in an amplitude variation of the oscillator voltage. During the positive half of the audio modulation-cycle the damping is greatest; hence the oscillation voltage is least. Conversely, during the negative half of the audio modulation cycle the damping is least, hence the oscillation voltage is greatest. Tube 24, which is termed the "Constant amplitude tube is shunted across the'oscillator tank cir cult, and presents a resistance load, or damping to the tank circuit. If the damping caused by tube 24 is made equal and opposite in phase to the damping caused by the control tube l 4 then the total damping over the cycle will be constant,

.ing across the tankcircuit, which will lower the Y tank voltage, the plate supply voltage is inand, hence, the oscillator voltage will lac-constant. This may be done by applying to the grid of tube 24 the-same audio modulation voltage as' that applied to the grid of tube l4. However,

the phase must be reversed 180 degrees so that when maximum damping due to tube l4 occurs, minimum damping due to tube 24 "will occur.

The amplitude of the voltage applied to the grid of tube M as well as the operating bias and tube type would be chosen to provide the correct damping.

In Fig. 2 there is shown the oscillator circuit, the quadrature tube network being omitted since it-can be exactly the same as that shown in Fig. 1. In this case constant oscillator amplitude is maintained by connecting tube 24 is shown. The cathode of the tube is connected to one end of choke coil 3 through a resistor 30. The plate 25 is connected to +3 terminal of the direct current source, while the control grid 26 is com nected to the lower end of resistor 30 through a grid leak resistor 3|. The grid 26 is connected creased. This will increase the tank voltage; hence, over the operating cycle 0! modulation voltage the oscillation voltage will be constant.

In Fig. 3 the modulation voltage isapplied to grid 50 of the oscillator tube 40. This in effect changes the Gm of the oscillator to hold theoscillator strength constant. If the Gm of an oscillator .tube is increased the oscillation strength will be likewise increased. Conversely, if the Gm is decreased the oscillation strength will be decreased. If the modulation voltage that is abplied to the quadrature tube grid is applied to a grid of the oscillator in the sam'e'phase, then as the damping due to the quadrature tube is increased, which lessens the oscillation strength, the Gm oi the oscillator will be increased. This increases the oscillation strength; hence, thetwo act in opposition which results ina constant, os cillator voltage. v

' While I have indicated and described several 4 systems Ior carrying'my invention into effect, it

to the modulation voltage source 2i (not shown both the functions of generating oscillations as well as compensating for amplitude variation. Thus, in Fig. 3, where again the quadrature network has been omitted, the numeral 40 designates an electron .discharge tube which has arranged between its cathode 4| and plate 42 four control grids. The grid 43 functions as the oscillator grid,;while the plate42- is connected to a source of positive potential through the radio frequency choke coil 44. The plate and of the coil is returned to ground through the capacity 45 The plate 42 then functions as the oscillator anode. The grids 46 and." are connected together, and are established at a positive potential by virtue of the connection through resistor 48 to the positive terminal end of coil 44. The grid 50, located between the positive screening grids 46 and 41 has applied to it the audio voltage from the modulation voltage source 2|. The, -voltageis applied through coupling condenser 60, the grid side of condenser being connected- 4 to ground through-the leak resistor 10. Here,

again, by applying audio voltage of correct phase and amplitude to grid 50 amplitude variation in thefre'quency modulated oscillation voltage developed across tank circuit 9-ll is substantially eliminated.

'will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications maybe made without departing from the scope of my invention, as set forth in the appended claims;

What I claim is:

1. In an oscillationgenerator of the type'comprising a tube having electrodes including a plate coupled to a source of supply voltage and to a network for producing oscillations of a predetermined frequency, an electron discharge device operatively associated with said network to provide a simulated reactive eflfect therefor, an audio modulation voltage source connected with said device for varying the magnitude of said reactive efiect thereby to frequency modulate said oscillations, and a second device operatively as sociated with said-network to provide compensation for amplitude modulation efiects introduced during frequency modulation" of the oscillations, and said second device beingconnected to said modulation source whereby amplitude modulation effects produced by said modulation source are substantially eliminated, said network includ- J ing an oscillation tank circuit, said electron dis-' charge device; having input and output .electrodes, a reactive path connected in shunt with with input and output electrodes, said output electrodes of said'last device being connected in series between said oscillator supply voltage source and oscillator tube plate, said modulating voltage being coupled to said input electrodes oi The constant amplitude tube 24 of Fig. 2 opersaid additional device. l-

2. In an oscillation generator of the type com-- prising an electron discharge device having a plurality of electrodes coupled in a reactive network for producing oscillations of a predetermined frequency, said tube having an additional electrode, an additional electron discharge device 6perative- 1y associated with said network to provide a simulated reactive eilect therefor, an audio modulation voltage source connected with said additional device for varying the magnitude of said reactive .effect thereby to frequency modulate said generated oscillations, and a second means opera-- lation source are substantially eliminated, said network including an oscillation tank circuit, said additional electron discharge device having input and output electrodes, a reactive path connected in shunt with said tank circuit and providing an alternating voltage which is in quadrature with the tank circuit voltage, means for impressing the quadrature voltage on said input electrodes, said output electrodes having their impedance existing therebetween connected across said tank circuit to provide said reactive eiiect, and said modulating voltage source being connected to said additional electrode of saidfirst electron discharge device to control its electron discharge thereby eliminating amplitude changes across said tank circuit.

3. In a wave length modulation system, an oscillation generator including a tube generator having a cathode, a grid and a plate coupled by a tuned tank circuit which determines in part at least the frequency of the oscillations generated, a reactance tube having an'anode, a cathode, and a control grid, means for impressing oscillating voltages from said tank circuit on the anode of said reactance tube, means for impressing Voltages from said tank circuit-on the control grid of said reactance tube, said impressed voltages being displaced substantially 90 degrees, whereby a reactive efiect is produced in said reactance tube, means coupling the impedance between the anode and cathode of said reactance tube in shunt to said tank circuit to supplement the reactance of the tank circuit by the reactance of said reactance tube, a source of modulating potentials coupled with an electrode of saidreactance tube to modulate the potential thereon in accordance with modulating potentials to thereby modulate the gain of the reactance tube and modulate the supplemental reactance supplied to said tank circuit to, wave length modulate the generated oscillations, and means for compensating for undesired amplitude modulation produced by modulating the wave length of said oscillations comprising an electron discharge device having an anode and cathode and having an input electrode variably coupled-to said source of modulating potentials, connections connecting the anode-to-cathode impedance of said deviceLin series with the plate-to-cathode impedance of the first tube, the impedance of to compensate for said undesired amplitude modulations.

4. In a wave length modulation system, an oscillation generator including a tube-having input and output electrodes, a source of direct current in a circuit including the output electrodes of said tube, a tuned tank circuit which determines in part at least the frequency of the oscillations generated connected with electrodes of said tube,

a reactance tube having an anode, a cathode, and a control grid electrode, means for impressing oscillating voltages from said tank circuit on the anode of said reactance tube, means for impressing voltages from said tank circuit on the control grid electrode of said reactance tube, said impressed voltages being displaced substantially 90 degrees, whereby a reactive efiec-t is produced in said reactance tube, means coupling the anode and cathode of said reactance tube to said tank circuit to supplement the reactance thereof by the reactance of said. reactance tube, a source of modulating potentials, connections thereto for modulating the potential on said control grid electrode of said reactance tube to thereby modulate its gain and modulate the supplemental reactance supplied to said tank circuit to wave length modulate the generated oscillations, and means for compensating for amplitude modulation produced during said wave length modulation including an electron discharge path in said first named circuit including the output electrodes of said generator tube, a control electrode in said path, and connections between said last named electrode and said source of modulating potentials for modulating the potential on said electrode substantially in phase with modulation of the potential on the control grid of said reactance tube.

5. In a wave length modulation system, an oscillation generator including a tube having a discharge path and electrodes associated with a tuned tank circuit which determines in part at least the frequency of the oscillations generated, a reactance tube having an anode, a cathode and a control grid electrode, means for impressing oscillating voltages from said tank circuit on the anode of said reactance tube, means for impressing Voltages from-said tank circuit on said control electrode of said reactance tube, said impressed voltages being displaced substantially degrees, whereby a reactive efiect is produced in said reactance tube, means coupling the anode and cathode of said reactance tube in shunt to said tank circuit to supplement the reactance thereof by the reactance of said reactance tube, a source oi modulating potentials coupled to said control grid for modulating the potential on said control grid electrode of said reactance tube to thereby modulate its gain and modulate the supplemental reactance supplied to said tank circuit to wave length modulate the generated oscillations, and means for compensating for amplitude modulation produced during said wave length modu-- lation including a discharge path, in series with said first discharge path, a control electrode in said second path, and connections from said source of modulating to'said last control electrode for applyingthereto modulating potentials of a phase substantially the same as the phase of the modulating potentials applied to the control electrode or said reactance tube.

.JOHNARANKIN. 

